1. Field of the Invention
This invention relates to polarisation controllers.
2. Description of Related Art
Polarisation controllers are used in optical communication systems utilising monomode optical fibres. In such systems, in order to enhance the receiver sensitivity compared to the direct detection methods used at present in optical fibre systems, it is desirable to use coherent detection techniques. Such techniques however require the matching of the state of polarisation of the optical signal at the output of the transmission fibre to that of the local oscillator beam at the receiver. The problem arises for long lengths of optical fibres, i.e. kilometres, that the state of polarisation of the light emerging from the fibre will vary with time due to the inherent birefringence of the fibre, defects and strains either inherent or introduced by bending etc. of the fibre, temperature and pressure changes along the length of the fibre etc.
Thus, for example, a linear polarised input beam will emerge from the fibre with elliptical polarisation.
Polarisation controllers must therefore be used in such systems in order to match the state of polarisation of the output and local oscillator beams. A number of polarisation controllers have been proposed over the years, a review of such controllers being given in Journal of Lightwave Technology, volume LT-3, No. 6 published in December 1985. These include electromagnetic fibre squeezers, rotatable fibre coils, Faraday rotators, electro-optic crystals, phase plates and rotatable fibre cranks. All these proposed polarisation controllers suffer from disadvantages, however, the fibre squeezers, 1 rotatable fibre coils, Faraday rotators and electro-optic crystals not being capable of coping with endless, unbounded variations in the state of polarisation, whilst the phase plates and rotatable fibre cranks suffer from a slow temporal response. Whilst electro-optic crystals, for example pairs of lithium niobate crystals, have been used in practical systems, these suffer from the additional disadvantage of high operating voltages, typically 160 to 260 volts, and the long optical path length through the crystals leading to high insertion losses of typically 3 to 6 dB.